AwssumePlugin - support more situations

[kmanning]

• Support both upper and lower case environment variable names for role ARN's
• First look for 'AWS_ROLE_ARN', then look for environment-specific. Hierarchy:
  • Allow parameters to take highest precedence. If I want to override any defaults, I should be able to set AWS_ROLE_ARN directly for an environment.
  • Allow environment-specific roles to take second precendence, as a means to manage lots of pipelines across a team.
• If nothing is found, just skip AwssumePlugin all together
• Also, fix AWS_DEFAULT_REGION

[kmanning]

I like the sound of this functionality. It seems to me like it would handle most common use cases, including the many teams that use some form of <environment>_AWS_ROLE_ARN as well as a lot of RE's infrastructure/utility jobs that are account-specific repos/pipelines and have an ARN hard-coded.

[kmanning]

Defect: There's fishy behavior with the AWS_REGION variable here. I know I've seen it fail in the past. Vipar has AWS_DEFAULT_REGION set, and awssume documentation only talks about AWS_DEFAULT_REGION.

https://github.com/manheim/awssume

[kmanning]

Creating separate issue - investigate behavior further there. Issue #130